1879. ] The Formation of Cape Cod. 553 
perhaps a half mile wide, and twice as long from north to south. 
This is enclosed on all sides by the hills and high plains, but 
contains no water, showing that the plane of saturation is very 
deep ; while copious springs at the north foot of the hills indicate 
that it falls in that direction. Without descending more than 
twenty feet below its highest point, the road next enters upon a 
plain of gravel and sand, and thence extends seven miles before 
crossing the first hollow, which is at Ashunet pond. Beyond this 
point it crosses numerous depressions that are or have been 
water courses; but there is no break in the continuity of the 
plains, which in about twelve miles descend by a gradual slope 
from the height of two hundred feet to sea-level. 
These plains of Cape Cod are further like those of Long Island, 
Martha’s Vineyard and Nantucket in being indented by narrow 
arms of the sea, which reach one to two miles inland, filling the 
lower end of long depressions that continue across the plains 
to the north, being either dry or occupied by small streams. The 
plains and valleys which thus generally border the terminal mo- 
raines on their south side appear to have been formed by the same 
floods which deposited the large amounts of modified drift along 
the edge of the ice-sheet. Much of their finer gravel and sand 
was carried forward by the descending currents, and spread in 
these gently sloping plains, while the valleys of drainage seem to 
ave been made by the same waters at their lower stages. 
The continuation of these valleys below our present sea-level 
calls up one of the most complex but at the same time most 
important and interesting questions connected with glacial geol- 
ogy. This feature shows plainly that when these valleys were 
formed the sea did not reach so high upon the land as now; and 
if we extend our inquiries we find that everywhere around the 
world the glacial period was marked by most extraordinary 
changes in the relative heights of land and sea. These remark- 
able oscillations, which had one extreme at the equator and the 
other at the poles, appear to have been changes in the level of the 
ocean. It seems not unlikely that an eighth part of the earth's 
Surface had become covered with ice, and if we consider a slope 
of one-half a degree to be needed to give it motion, an estimate 
of four miles for its average depth does not seem to be too great. 
! These valleys on Long Island have been described by Mr. Elias Lewis, Jr., in 
American Fournal of Science and Arts, 34 series, Vol. XII, pp. 142-146 an 215. 
